MEDICINE LAKE VOLCANO: RESULTS OF GEOLOGIC MAPPING AND ARGON DATING

Medicine Lake volcano (MLV) is a broad shield lying east of the main Cascade axis. It covers ~2000 km² in northern California NE of Mt. Shasta; estimated volume is ~600 km³. Mapped Medicine Lake lavas are predominantly mafic. Basalt covers 66%, basaltic andesite 13%, and andesite 15%, whereas exposed rhyolitic and dacitic lavas are rare and total only 6%. By contrast, drill hole data reveal a volcano with a core consisting of 50-60% silicic flows, domes, and minor tuffs mantled by an overlay of predominantly mafic lavas. Many of these older silicic units are not exposed at the surface. Ar dating of most that are exposed plus correlation of drill hole units and a limited number of dates on drill hole samples indicate that the surprisingly silicic core of the volcano dates from about 0.48 to 0.30 Ma. Dates on the stratigraphically lowest mafic lavas at MLV fall into this time frame and indicate that volcanism at MLV began at ~0.5 Ma. However, the bulk of the mafic shield-forming lavas are younger than 0.3 Ma. Rhyolites were scarce after 0.3 Ma until late Holocene time, when they make up about 20% of the erupted post-glacial volume. The history of the volcano has been punctuated by distinctive episodes including a dacite episode around 0.2 Ma that apparently culminated in eruption of the tuff of Antelope Well (Andesite Tuff of Anderson, 1941), which is the volcano’s only ash-flow tuff. The age of this widespread marker unit is constrained to be about 0.18 Ma. At ~0.1 Ma, high-Na andesite and dacite that is compositionally unique in MLV’s history built the N, S, and E rims of the caldera. Eruption of these lavas was followed soon after by several large basalt flows such that the combined area covered during this episode amounts to nearly 1/3 of the area of MLV. Post-glacial eruptive activity had been previously documented as strongly episodic. New evidence indicates that rhyolites with SiO₂ contents >73.1 wt. percent erupted in the late Holocene for the first time since 0.3 Ma.